The present invention is directed to a lossy dielectric cap that dissipates electric fields that may be used near current transmission or conduction paths in a probing head of an electrical test probe.
FIG. 1 shows a probing system that includes an electrical test probe 20 for providing an electrical connection between electrical components 22 and testing instruments 24. An electrical test probe 20 generally consists of a probing head 30, a cable 32, and a testing instrument connector 34. The probing head 30 may have an integral or replaceable probe tip 40 that is suitable for making an electrical contact with electrical components 22. The testing instrument connector 34 is suitable for connecting to a testing instrument 24. If the probe tip 40 is replaceable, generally the probing head 30 will have a socket 38 or other connection mechanism for mating with the probe tip 40. The probing head 30 is attached to a first end of the cable 32 and the testing instrument connector 34 is attached to the opposite end of the cable 32.
As current flows through wires and electrical components, it generates electromagnetic fields. Some of the energy radiates around the wires and electrical components. At certain frequencies, the fields can reflect and bounce back into the wires and electrical components. This process tends to create a varying response with respect to frequency that is undesirable. Ideal test probes have a perfect curve frequency response (shown as a flat line on a frequency response graph) in which voltage in is equal to (or proportional to) voltage out.
FIG. 2 shows a frequency response graph of the exemplary 7.5 GHz bandwidth probe shown in FIG. 3. This exemplary probe has adjustable dual tip technology that allows the user to set the spacing of the probe tips in a continuously variable fashion. Within its housing 48, the probe head 30 in FIG. 3 includes a crisscross spring with a “U”-shape over molded on the free ends of the spring 50(a), 50(b). The probe head 30 has an elongated cast shell 52 that houses the bulk of a hybrid 54 with a custom amplifier. The 50-Ohm transmission line input 56a, 56b, 56c comes out of the elongated housing, arches up, splits at a 90 degree angle and bends into the “U”-shaped channels where they are fixed or glued. A thin 0.02 inch FR4 backer 58 backs the flex. As the backer 58 emerges from the “U,” a resistor 60 is in series with the transmission line 62 and tip 40. A small notch 64 on the FR4 backer 58 accommodates a grounding tether 66. The grounding tether 66 may be soldered or otherwise attached between the series resistor 60 and the 50-Ohm transmission line 56a, 56b, 56c. The 7.5 GHz bandwidth rating is typical for the probe as a stand-alone device. As an ideal frequency response is flat, the large increase in signal amplitude at high frequencies (peaking) shown in FIG. 2 would be understood as undesirable in a frequency response.
Much of the undesired variation in frequency response is due to the fact that some electromagnetic energy radiates into space from the probe tips. This energy can couple back onto the probe circuitry after the attenuating resistor, increasing or decreasing the signal level depending on the phase of the radiated path.
It has long been known that ferrite material (e.g. ferrite caps) can be used to dissipate, absorb, and/or dampen magnetic fields. In the exemplary probe head 30 of FIG. 3, ferrite material is shown as 68. Ferrite materials attempt to resolve problems associated with magnetic fields. When placed near metal conductors, these materials can attenuate the magnetic fields created by high frequency current flow, and thus reduce radiated fields. One problem with the ferrite materials is that they do not solve the problems associated with electrical fields. Another problem with the ferrite materials is that since they are placed near conductors to dampen the magnetic fields, they can also change the electrical characteristics of the conductors. For example, placing a ferrite bead around a probe tip will increase the tip inductance at the same time as reducing the radiated field. Test probes with ferrite materials do not have perfect frequency responses.
Other material such as conductive foam (e.g. foam with slightly conductive properties) and conductive films have also been used for solving problems with electromagnetic fields. None of these products have provided satisfactory results.